alkane

College of Dentistry Lecture Three
Organic Chemistry
1
Assistant Lecture
Najlaa Nassir
Alkanes – An Introduction
Hydrocarbon: Compound composed of only carbon and hydrogen
Saturated Hydrocarbons: Compound with only single bonds
Unsaturated Hydrocarbons: Compounds with AT LEAST one double or
triple bond
Alkane: Compound composed of only carbon and hydrogen and single
bonds.
Alkanes are aliphatic hydrocarbons having only C?H and C?C ?-bonds.
They can be cyclic or acyclic.
Acyclic Alkanes: Compound composed of only carbon and hydrogen in a
chain
•Acyclic alkanes have the molecular formula CnH2n+2 .
They are also called saturated hydrocarbons because they have the maximum
number of hydrogen atoms per carbon.
•Cyclic alkanes contain carbon atoms joined into a ring. They have molecular
formula CnH2n .In a molecule, atoms that are not carbon or hydrogen are
called Heteroatoms (such as N, O). Cyclic structure containing heteroatoms
are called heterocyclic.
College of Dentistry Lecture Three
Organic Chemistry
2
Unbranched Alkanes :-
Alkanes with unbranched carbon chains are also known as normal alkanes or
n- alkanes.
The first four n-alkanes:

This is an example of a family of compounds known as homologous series
(each member differs from the next by the addition of one methylene group
(CH2).
Branched Alkanes
As the number of carbons of an alkane increase beyond three, the number of
possible structures increases. An alkane with molecular formula C4H10 for
example
has two different ways to connect atoms together:
College of Dentistry Lecture Three
Organic Chemistry
3
These are examples of constitutional isomers: Compounds that have the same
molecular formula but different connectivity
There are three constitutional isomers with molecular formula C5H12.
Carbons in alkanes or other organic compounds can be classified as primary
(1°),)secondary (2°), tertiary (3°), and quaternary:
•a primary carbon is bound to one other carbon
•a secondary carbon is bound to two other carbons
•a tertiary carbon is bound to three other carbons
•a quaternary carbon is bound to four other carbons.
Hydrogen s can also be classified as 1°, 2°, 3°depending on the type of
carbon atom to which they are bonded:
•primary H’s are attached to primary carbons
•secondary H’s are attached to secondary carbons
• tertiary H’s are attached to tertiary carbons.
College of Dentistry Lecture Three
Organic Chemistry
4

Naming Alkanes:
The name of every organic molecule has 3 parts:
? The parent name indicates the number of carbons in the longest
continuous chain.
? The suffix indicates what functional group is present.
? The prefix tells us the identity, location, and number of substituents
attached to the carbon chain.
Naming Substituents:
•Carbon substituents bonded to a long carbon chain are called alkyl
groups.
•An alkyl group is formed by removing one H atom from an alkane.
•To name an alkyl group, change the –ane ending of the parent
alkane to –yl. Thus, methane (CH4) becomes methyl (CH3-) and
ethane (CH3CH3) becomes ethyl (CH3CH2-).
Prefix Parent Suffix
College of Dentistry Lecture Three
Organic Chemistry
5
Where are the substituents? How many carbons? What family?
Nomenclature of Cycloalkanes:
General formula: CnH2n
Nomenclature: Add prefix "cyclo" to alkane name
•Substituents are named similar to the way they are in straight-chain alkanes.
•If the ring has two different substituents, they are cited in alphabetical order,
and the number one position is given to the substituent cited first.
Physical Properties of Alkanes:
Alkanes contain only nonpolar C?H and C?C bonds, and as a result
they exhibit only weak van Der Waals forces.
College of Dentistry Lecture Three
Organic Chemistry
6
•Alkanes have low bp’s and mp’s compared to more polar compounds
of comparable size.
•Bp and mp increases as the number of carbons increases because of
increased surface area.
•Bp decreases with increased branching because of decreased surface
area.
Reactions of alkane:-
1- Oxidation –Reduction reaction:
Oxidation increasing the number of C O bonds.
Reduction increasing the number of C H bonds
CH4 + 2O2 CO2 + 2H2O + heat (energy)
(CH3)3CCH2CH(CH3)2 + 25O2 8CO2 +9H2O + eneargy
2,2,4,-trimethylpentane oxidized product
(isooctane)
Reduced starting material